1. Field of The Invention
This invention pertains to a method and an apparatus that forms ablated features in substrates exhibiting more accurate shapes with less shape distortion, and is especially applicable to polymer substrates.
2. Description of the Related Art
The laser ablation of features on polymer materials using a mask and imaging lens system is well known. In this process, features on the mask are illuminated with laser light. The laser light that passes through the transparent features of the mask is then imaged onto the substrate such as a polymeric film where the ablation process occurs.
FIG. 1 illustrates a basic layout of a conventional excimer laser machining system. Typically, the system is controlled by a computer with an interface to the operator of the system. The computer controls the firing of the pulsed laser system and a servo system. The function of the servo system is to position the mask and substrate chuck for proper registration of the laser milled pattern with respect to other features on the substrate. For this purpose, a vision system (not shown) is often interfaced to the computer system. The servo system or computer may control an attenuator module, to vary the amount of UV radiation entering the system. Alternatively, the laser pulse energy may be varied by adjusting the laser high voltage or a control set point for energy, maintained by the laser's internal pulse energy control loop.
The UV beam path is indicated in this figure with arrows (schematic only as these are not intended to be actual raypaths, which are not typically parallel) which show the flow of UV energy within the system. The UV power originates at the pulsed excimer laser. The laser typically fires at 100-300 Hz for economical machining with pulses that have a duration of about 20-40 nanoseconds each. The typical industrial excimer laser is 100-150 watts of time average power, but peak powers may reach megawatts due to the short duration of the pulse. These high peak powers are important in machining many materials.
From the output end of the laser, the UV energy typically traverses an attenuator; however, this is an optional assembly not present in all laser machining systems. The attenuator performs either or both of two possible functions. In the first function, the attenuator compensates for the degradation of the optical train. The attenuator thus used, allows the laser to run in a narrow band of pulse energies (and hence a restricted range of high voltage levels), allowing for more stable operation over the long term. With new optics in the system, the attenuator is set to dissipate some of the power of the laser. As the optics degrade and begin to absorb energy themselves, the attenuator is adjusted to provide additional light energy. For this function, a simple manual attenuator plate or plates can be used. The attenuator plates are typically quartz or fused silica plates with special dielectric coatings on them to redirect some of the laser energy toward an absorbing beam dump within the attenuator housing.
The other possible function of the attenuator is for short term control of laser power. In this case, the attenuator is motorized with either stepper motors or servo system, and the attenuator is adjusted to provide the correct fluence (energy per unit area) at the substrate for proper process control.
From the attenuator, the UV energy propagates to a beam expansion telescope (optional). The beam expansion telescope serves the function of adjusting the cross sectional area of the beam to properly fill the entrance to the beam homogenizer. This has an important effect on the overall system resolution by creating the correct numerical aperture of illumination upon exit from the homogenizer. Typical excimer laser beams are not symmetric in horizontal vs. vertical directions. Typically, the excimer beam is described as "top hat-gaussian," meaning that between the laser discharge direction (usually vertical), the beam profile is "top hat" (flat top and drops off sharply at the edges). In the other direction, the beam has a typical intensity profile that looks qualitatively gaussian, like a normal probability curve.
The telescope allows some level of relative adjustment in the distribution of power in these directions. This reduces (but does not eliminate) distortion of the pattern being imaged onto the substrate, due to differing numerical apertures (the sine of the half angle of the cone of light) in these orthogonal beam directions, since imaging resolution is directly a function of numerical aperture.
Between telescope and homogenizer we have shown a flat beam folding mirror. Most systems, due to space limitations, will contain a few such mirrors to fold the system into the available space. Generally, they may be placed between components, but in some areas, the energy density or fluence can be quite high. So mirror locations are carefully chosen to avoid such areas of high energy density. In general, the designer of such a system will try to limit the number of folding mirrors in order to minimize optics replacement cost and alignment difficulty.
The UV light next enters the beam homogenizer. The purpose of the homogenizer is to create a uniform intensity of the illumination field at the mask plane. It also determines the numerical aperture of the illumination field (the sine of the half angle of the cone of light impinging on the mask), which as stated above, has an impact on overall system resolution. Since certain parts of the excimer beam are hotter than others, uniform illumination requires that the beam be parsed into smaller segments, and stretched and overlaid at the mask plane. Several methods for this are known in the art, with some methods being based on traditional refractive optics, e.g., as disclosed in U.S. Pat. Nos. 4,733,944 and 5,414,559, both of which are incorporated herein by reference. Homogenization may also be based on diffractive or holographic optics, as in U.S. Pat. No. 5,610,733, both of which patents are incorporated by reference. Alternatively, it may be based on continuous relief microlens arrays ("Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing", Nikoladjeff, et. al, Applied Optics, Vol. 36, No. 32, pp. 8481-8489, 1997).
From the beam homogenizer the light propagates to a field lens, which serves to collect the light from the homogenizer and properly couple it into the imaging lens. The field lenses may be simple spherical lenses, cylindrical lenses, anamorphic or a combination thereof, depending on the application. Careful design and placement of field lenses are important in achieving telecentric imaging on the substrate side of the lens.
The mask is typically placed in close proximity to the field lens. The mask carries a pattern that is to be replicated on the substrate. The pattern is typically larger (2-5.times.) than the size desired on the substrate. The imaging lens is designed to (de)magnify the mask in the course of imaging it onto the part. This has the desired property of keeping energy density low at the mask plane and high at the substrate plane. High de-magnification usually imposes a limit on the field size available at the substrate plane.
The mask may be formed from chromium or aluminum coated on a quartz or fused silica substrate with the pattern being etched into the metallic layer by photolithography or other means. The reflecting and/or absorbing layer on the mask may comprise a sequence of dielectrics layers, such as those disclosed in U.S. Pat. Nos. 4,923,772 and 5,298,351, both of which are incorporated herein by reference.
The purpose of the imaging lens is to demagnify and relay the mask pattern onto the substrate. If the pattern is reduced by a factor of M in each dimension, then the energy density is raised by M2 multiplied by the transmission factor of the lens (typically 80% or so). In its simplest form, the lens is a single element lens. Typically, the lens is a complex multi-element lens designed to reduce various aberration and distortions in the image. The lens is designed with fewest elements to accomplish the desired image quality in order to increase the optical throughput and to decrease the cost of the imaging lens. Typically, the imaging lens is one of the most expensive parts of the beam train.
The imaging lens creates a de-magnified image of the pattern of the mask on the substrate. Each time the laser fires, an intense patterned area is illuminated on the substrate. As a result, etching of the substrate material results. Many substrate materials may be so imaged, especially polymeric materials. Polyimides under various trade names are the most common for microelectronic applications and inkjet applications.
This system described in FIG. 1 is a "typical" system. For non-demanding applications, the system can be further simplified and still produce parts, but with some sacrifice in part tolerances, repeatability, or both. It is not unusual for systems to make some departure from this typical architecture, driven by needs of the application. For example, in U.S. Pat. No. 4,940,881, incorporated herein by reference, it is disclosed that the insertion of a rotating refractive element between the imaging lens and the mask, will provide some level of control over the shape of the ablated hole.
There are many applications for laser ablation of polymeric materials. Some applications or portions thereof are not demanding in terms of tolerances, e.g., electrical vias, and the emphasis is on small size for high density and low cost. Other applications require very demanding tolerances and repeatability. Examples are fluid flow applications such as inkjet printhead nozzle manufacture and manufacture of drug dispensing nozzles. In these applications, the exact size, shape, and repeatability of manufacture are critical. The detailed architecture of the system is critical to obtaining tight tolerances and product repeatability. In addition, process parameters and the optical components all play important roles in obtaining the tightest possible tolerances, down to the sub-micron level.
The object of this invention is a means for controlling the shape of the laser ablated feature at the sub-micron level for these demanding applications, and limiting shape distortion. In this invention, we show two means to control the shape of ablated features such as holes or nozzles, by the agency of beam polarization effects. The laser beam itself may be randomly polarized, or may have a high degree of polarization, e.g., linear polarization, or another more general polarization state, depending upon the laser resonator configuration. Even if the laser is randomly polarized, the effect of beam attenuators and folding mirrors introduces partial polarization. Other system components, depending upon their details, may also impart some polarization. As round holes are particularly important for many electronics applications, we will focus primarily on ovality control for round holes, however, the invention is equally applicable to other ablated feature geometries.
When partially polarized light impinges on the sidewall of a partially ablated feature, as it is being formed, the reflectivity of the incident UV light is a function of polarization and the angle of incidence of the light. UV energy that is reflected, is by definition, not available for absorption and ablation. The result is that the amount of absorbed radiation varies azimuthally around a hole or nozzle as it is being formed, with the result that polarization induces ovality into the ablated hole or nozzle. Simple geometric considerations show that features of other shapes will also be distorted by this effect.